William Pitt the Younger (28 May 1759 — 23 January 1806) was a British politician of the late eighteenth and early nineteenth centuries. He was Chancellor of the Exchequer from 1782—1783, 1783—1801 and 1804—1806. He was prime minister of the United Kingdom from 1783 to 1801, and again from 1804 until his death. He is known as "William Pitt the Younger" to distinguish him from his father, William Pitt the Elder, who also served as prime minster. He became the youngest prime minister in 1783 at the age of 24. During his time as prime minister, the kingdom of Ireland joined with Great Britain and both kingdoms became the United Kingdom of Great Britain and Ireland on 1 January 1801. Pitt continued to be prime minister in the new Parliament of the United Kingdom. Known as "William Pitt Jr.", unlike his father and the Prime Minister of the United Kingdom, William Pitt Sr.

The period of William Pitt Jr.'s presidency was during the reign of King George III and was accompanied by significant events in Europe, such as the French Revolution and the Napoleonic Wars.

Pitt is often considered a Conservative party, but he considered himself non-partisan and generally opposed a strict party political system.